The present invention relates to a three-dimensional knitted noble metal gauze suitable for carrying out catalytic gaseous reactions, in particular for the catalytic oxidation of ammonia with air to obtain nitric acid.
The use of noble metal catalysts in the form of gas-permeable woven fabrics is a proven method for operating certain catalytic processes at optimum yields. Catalyst gauzes for ammonia oxidation processes are well known and are generally formed of a platinum or a platinum alloy wire. A known example is the use of platinum/rhodium catalyst gauzes in the combustion of ammonia for nitric acid production. This reaction can be represented by the equations below: EQU 4NH.sub.3 +5O.sub.2 .fwdarw.4NO+6H.sub.2 O EQU 2NO+O.sub.2 .fwdarw.2NO.sub.2 EQU 3NO.sub.2 +H.sub.2 O.fwdarw.2HNO.sub.3 +NO
Woven gauzes based on palladium are used in ammonia oxidation plants to act as "getters" or collectors of platinum lost by the chemical action from the catalyst gauzes.
Catalyst gauzes are normally produced on weaving looms from thin noble metal wires. It is also known to knit metal wires into knitted metal fabrics of ferrous alloys, e.g. stainless steel. Uses proposed for such fabrics include spray separators for evaporators, for conveyor belting, filter components and the like. A process for the production of catalyst gauzes from noble metal wires by knitting has recently been disclosed by EP-A-0 364 153 corresponding to U.S. Pat. No. 5,188,813. This document also mentions three-dimensional knitting, but no further details concerning the structure of these knitted fabrics are given.
As stated therein, it has it has also been suggested to sputter metal onto a woven or knitted synthetic fabric (Japanese publication 61282584) to obtain a heat reflective curtain material. A metal-plated optical fiber knitted fabric has been suggested as an electromagnetic shielding material. U.S. Pat. No. 4,609,923 discloses a knitted gold-plated tungsten wire for use as a radio frequency reflector; such a material possesses sufficient tensile strength and other properties to permit knitting. In the art of ammonia oxidation, Great Britain Patent 1,411,800 mentions the use of knitted heat resistant alloy wire as a support element for conventional woven catalyst gauzes, but does not contemplate a knitted catalyst fabric.
Great Britain Specification 2,088,914A discloses fabrics from brittle reinforcing fibers, especially weaving carbon fibers. The possibility of applying the technique to brittle metal fibers and tungsten is mentioned. Precious metal fibers are not brittle in the manner of carbon fibers and are known to be capable of being woven.
Great Britain Specification 549,418 discloses woven or knitted iron, steel or brass fabrics in which a textile fiber such as cotton wool or rayon is incorporated with the metal wire to act as an absorbent for oil in an air filter for engines.
The techniques for weaving metal screen, meshes or gauzes is well known including composition of the wires or thread, thickness of thread, thickness of gauzes etc.
Knitted gauzes, like all catalyst gauzes, have the advantage of a large catalytic surface area combined with elevated mechanical strength and simple production techniques. However, the relatively large losses of noble metal during the reaction and the associated short service life of the gauzes are disadvantageous. Moreover, if the catalyst gauze wires are predominantly arranged perpendicular to the direction of flow of the gas stream, the course of the reaction becomes unstable on the wire surfaces with corresponding temperature oscillations. This brings about a non-linear course of the reaction and has a negative effect on product yield and the service life of the gauzes.